This invention is in the field of curved magnetic latches or holding devices specifically in the form of a magnetic drum or cylinder for use in holding printing plates or the like. More particularly, the invention is directed toward magnetic cylinders which are formed by placing magnetic elements into channels or recesses or pockets on the outer surface of the cylinder.
U.S. Pat. No. 5,627,505 by Iwaszek describes a magnetic cylinder in which magnetic elements comprising a number of bar magnets with intermediate pole pieces in close intimate contact with one another are located in axial extending recesses or slots or pockets or channels formed on the surface of a cylindrical drum to provide the magnetic field for the magnetic cylinder. U.S. Pat. No. 5,898,352 by McEachern, et al. relates to the same type of magnetic drum but introduces a foraminate layer at the bottom of the channel for the dual purpose minimizing air pockets in any adhesive that is used and for serving as a magnetic insulator between the magnetic elements and the cylinder core. Another U.S. Pat. No. 5,938,579 by Cavazos deals with a similar magnetic cylinder which uses nonmagnetic spacers in the channels to serve as magnetic insulators for the magnetic elements located in the channels.
The ""505 patent uses a nonmagnetic stainless steel as the material for the cylinder because of its low or nonmagnetic permeability so as to minimize or eliminate magnetic field leakage. The ""352 patent uses a foraminous layer at the bottom of the channels to minimize adverse effects of air bubbles in any adhesive and provide some degree of magnetic insulation. The ""579 patent utilizes spacers in the channels for magnetic insulation. The ""352 patent suggests the possibility of making a cylinder out of less costly magnetically permeable mild or tool steel and the ""579 patent cylinder is made out of soft mild steel.
The present invention is aimed at providing a magnetic cylinder constructed similar to those in the aforementioned U.S. patents in which magnetic elements are placed in channels or recesses on the outer surface of the cylinder. The cylinder is constructed with a core made out of soft mild steel or tool steel which is magnetically permeable with a sleeve or a shell over the outer surface of the core. The sleeve is made out of hard stainless steel or aluminum which has a very low magnetic permeability. The channels or pockets are formed in the shell so that the magnetic elements can be placed in these channels without the need for spacers or other types of magnetic insulating devices. In this fashion then the bulk of the cylinder is made out of the much lower cost mild steel or tool steel and only the shell, which generally constitutes a small volume of the cylinder, is made out of the more costly stainless steel with the resulting advantage of eliminating magnetic field leakage. The savings in eliminating magnetic insulating spacers offsets any cost involved in forming the cylinder out of a shell of stainless steel over a soft steel core. It may also be less costly to machine or otherwise form the recesses in the outer shell or sleeve layer. If aluminum is used there may also be a weight-saving benefit.